define_extension :sources, :persistent_one_time_wrapper do
  task :sources do
    unless revision_build.checked_out
      execute_previous
      revision_build.checked_out = true
    end
  end
end

define_extension :sources, :one_time_wrapper do
  task :sources do
    unless fetch(:_done, false)
      execute_previous
    end
    set :_done, true
  end
end
